1. Field of the Invention
The present invention relates to a field effect transistor, and particularly to a field effect transistor used in a high output amplifier of a mobile communication base station, an artificial satellite or the like.
2. Description of the Related Art
The quality of transferred sound, and the like, as well as merely, reduction in size and weight, become important point in the development of a recent portable terminal such as a cellular phone. There is a need to transfer a signal with less distortion upon improving sound quality. It is important for a high output power amplifier such as a mobile communication base station to provide a less reduction in distortion at its amplifying characteristic in addition to gain as one important item for achieving the above. Similarly, a satellite's onboard high output amplifier used in satellite communications or the like needs to be simply brought into not only less weight and a gain increase but also into a less reduction in distortion at its amplifying characteristic.
These high output power amplifiers make use of lots of field effect transistors using compound semiconductors in conjunction with their speed properties or speedability. For instance, a MESFET (MEtal Semiconductor FET) using a GaAs channel has been widely used.
As a known example of the MESFET using GaAs, an example has been disclosed which shows an MESFET configured, with a gate length ranging from 40 nm to 300 nm, using a laminated structure wherein a buffer layer corresponding to an undoped Al0.3Ga0.7As layer whose thickness is 200 nm, is provided on an undoped GaAs layer having a thickness of 700 nm, and an Si-doped GaAs channel layer (active layer) whose thickness is set as 50 nm with recesses provided thereon and having a doping density of 4×1018 cm−3, is provided thereon (see, for example, “Short-Channel Effects and Drain-Induced Barrier Lowering in Nanometer-Scale GaAs MESFET's”, James A. Adams et Al., IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 6, JUNE 1993, pp. 1047-1052; the left-hand section in Page 1048).
The conventional MESFET is provided with a buffer layer in close contact with an n-GaAs channel layer, which corresponds to an undoped AlGaAs layer higher than the n-GaAs channel layer in energy level at conduction band. Owing to such a configuration, the diffusion of electrons in the n-GaAs channel layer into the buffer layer allows prevention of at least expansion of an electron distribution in the vicinity of the interface between the n-GaAs channel layer and the buffer layer and deterioration of mutual conductance (gm) and drain conductance (gd).
In the present configuration, however, an electron distribution in a thickness direction of the n-GaAs channel layer simply shows a flat distribution. Therefore, when a bias corresponding to a negative voltage is applied to a gate electrode to thereby spread a depletion layer into the n-GaAs channel layer, the depletion layer is relatively small and hence no particular problem occurs in the migration of electrons, where the absolute value of the bias corresponding to the negative voltage is low. However, a problem arises in that when the absolute value of the gate bias corresponding to the negative voltage becomes large and thereby reaches near pinch off, degradation of linearity of mutual conductance (gm) and deterioration of drain conductance (gd) occur.
Incidentally, it has already been pointed out that it is effective to increase the density of electrons in the vicinity of an interface between a channel layer and a buffer layer and on the channel layer side in order to ensure the linearity of mutual conductance (gm) (see “Graded Channel FET's: Improved Linearity and Noise Figure”, R. E. WILLIAMS et Al., IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-25, NO. 6, JUNE 1978, pp. 600-605; the left-hand section in Page 602).